1. Field of the Invention
This invention relates generally to the field of geophysical prospecting. More particularly, the invention relates to the field of marine seismic data acquisition.
2. Description of the Related Art
To perform a three-dimensional (3D) marine seismic survey, a plurality of marine seismic streamers are towed at a preset depth, typically between 4 and 25 meters, behind a surface survey vessel. Each seismic streamer, also referred to as a streamer cable, is typically several thousand meters long and contains a series of seismic sensors and associated analog-to-digital signal converter electronics distributed along the streamer length. The streamer cables comprise a series of individual segments, called streamer sections, each typically 75 to 200 meters long. The survey vessel also tows one or more seismic sources, for example air guns or water guns, but most commonly consisting of arrays of air guns. Acoustic signals generated by the seismic sources are transmitted down through the water column and several more kilometers down into the subterranean formations. Parts of the signals are reflected from the interfaces between various strata, due to differences in the acoustic impedance between different rock formations. The acoustic signals reflected from the subterranean formations are detected by the seismic sensors located within the streamers. The acquired seismic signals are digitized and sent via a main telemetry link to the survey vessel for data processing onboard or later processing onshore. The processed data is used for estimating the subterranean formation structure and possible hydrocarbon content.
FIG. 1A illustrates a top schematic view of an ideal case, with no cross-currents, of a 3D marine seismic survey using towed streamers. A seismic survey vessel 1 tows a relatively small seismic tow system which comprises an active source consisting of three air gun arrays 2, and a spread of four streamer cables 3. The streamer cables 3 extend from streamer separation doors (also referred to as deflectors) 4 at the front of the spread to tail buoys 5 at the rear. In this ideal case, the streamer cables 3 all extend behind the vessel 1 in unrealistically straight and equally spaced lines parallel to the vessel track and to each other. FIG. 1B illustrates a top schematic view of a more realistic case of a 3D marine seismic survey using towed streamers, showing the typical effects of cross-currents on the streamer spread. The separations between the streamers 3 are no longer constant and the positions of the tail segments deviate from the vessel track. This deviation effect is called “feathering”. The tail segments of the streamers can, in some survey areas, deviate significantly from the vessel track due to the cross-currents along the tow spread.
For correct seismic imaging of the sub-bottom beneath the survey area, it is important to accurately determine the position of both the air gun sources and the seismic receivers. The seismic sources are towed relatively closely behind the survey vessel and are easier to control than the streamer spread. Streamer spreads typically consist of 8 to 12 independently towed streamer cables, with each streamer being 3 to 8 kilometers long. However, the trend is to deploy even more and longer streamers, such as up to 20 streamers of approximately 12 kilometers length. Accurate determination of streamer positions is also important in avoiding high risk operational situations such as streamer tangling. The tangling can be caused by strong water currents in the sea when more than one cable is hooked up and connected. Resolving such tangling scenarios is complex and may expose the seismic crew to hazardous in-sea operations, in addition to being quite costly.
Methods for determining streamer positions have included the use of devices such as Global Positioning System (GPS) receivers, magnetic compasses (also referred to as magnetic heading sensors), acoustic transmitters, conventional streamer hydrophones, or acoustic receivers specifically dedicated to the position-determining task.
U.S. Pat. No. 4,231,111, “Marine Cable Location System”, issued to Walter P. Neeley on Oct. 28, 1980, discloses a method for determining streamer positions that distributes magnetic compasses along the streamer cables at regular intervals and employs the heading information from these compasses to model the shape and orientation of each cable. However, the externally-mounted compasses are sometimes lost due to streamer entanglement or other impact situations, and the compasses create flow noise on neighboring seismic sensors. Additionally, the compasses are charged by batteries which need to be replaced at certain intervals and the compasses have to be re-calibrated in the factory after any repairs or changes.
U.S. Pat. No. 5,761,153, “Method of Locating Hydrophones”, issued to Vassilis N. Gikas, Paul A. Cross, an Asiama Akuamoa on Jun. 2, 1998, discloses a method for determining streamer positions that employs both magnetic compasses and acoustic transceivers, including both transmitters and receivers. The transmitters and receivers, just as the magnetic compasses, are externally attached to the streamer cables and seismic sources, powered by batteries and communicate via inductive coils located within the streamers. Coded ultrasound signals are transmitted between the transceivers. The transceivers measure distances between transmitters and receivers and enable both the shape of the towed arrays to be determined and the relative positions of the seismic sensors to be estimated. In methods exemplified by Gikas et al. '153, the transmitters and receivers are only placed at the front, center and tail of the streamer spread, due to the high cost of the acoustic transceivers. The magnetic compasses are then used to determine the streamer positions between the transceiver locations. This method still has the problems associated with externally mounted magnetic compasses, as discussed above with respect to Neeley '111.
U.S. Pat. No. 4,992,990, “Method for Determining the Position of Seismic Streamers in a Reflection Seismic Measuring System”, issued to Jan-Åge Langeland, Stein ÅSheim, Bjorn Nordmoen, and Erik Vigen on Feb. 2, 1991, discloses a method for determining streamer positions that deploys acoustic transceivers throughout the complete streamer spread. Langeland et al. '990 employs acoustic transceivers positioned on the seismic vessels, tail buoys, a float towed near the front of the streamers, the stretch sections at the front and rear of the streamers, and possibly in the active sections of the streamers. The transceivers operate in the frequency band of 25 to 40 kilohertz (kHz). Starting with two known positions, preferably on the tow vessel and the float, the positions of the other transceivers are determined by trilateration of the transit times (and hence the distances) between the transceivers to form a solvable triangular network. However, employing the seismic acquisition receivers to determiner streamer positions can cause problems when some of the seismic acquisition receivers are unavailable, due to mechanical or electrical failure in the streamers or elsewhere in the tow system. Additionally, this method has the problems associated with externally mounted transceivers, as in Gikas et al. '153 above.
U.S. Pat. No. 4,912,682, “Point Location Determination At or Close to the Surface”, issued to John P. Norton, Jr., Michael A. Hall, and Ian N. Court on Mar. 27, 1990, discloses a system in which ultrasonic sonar transmitters are positioned along a streamer, preferably at 300 meter intervals, and seismic receivers are positioned along a streamer, preferably at 100 meter intervals, so that there are three times as many receivers as transmitters. The transmitters emit a unique tone burst or series of tone bursts or a continuously varying tone as in a chirp signal. The signal is transmitted in the 50 to 150 kHz band. The receivers are time-gated to only register the signal during a set time period, to limit the number of receivers apportioned to each transmitter, preferably to seven. The elapsed time for each transmitter-receiver pair is converted to distance by multiplying by the local speed of sound in water, which is assumed constant. The positions of the front of each streamer are presumed known and the positions of the transmitters and receivers are solved sequentially down the streamer lengths by a variation of coordinates technique. In this technique, a set of equations in the additive correction terms is reduced to normal equations by least squares and then solved. From the relative spacing between transmitters and receivers, their relative positions can be determined. However, employing the seismic acquisition receivers to determine streamer positions and attaching the transmitters and receivers externally to the streamer leads to the same problems discussed in Langeland et al. '990, above.
U.S. Pat. No. 6,839,302 B2, “Acoustic Emitters for Use in Marine Seismic Surveying”, issued to Peter Austad and Rolf Rustad on Jan. 4, 2005, discloses a method that avoids the drawbacks of externally-mounted equipment by putting the transmitters or receivers in special sections that can be inserted between conventional streamer sections. Austad et al. '302 describes an acoustic emitter that can be inserted between adjacent streamer sections. The emitter comprises an annular housing containing an annular piezoelectric emitting element, which is protected from bending by surrounding flexible barrel stave members. The emitter is adapted to operate in a frequency range up to 10 kHz. However, locating the transmitters in additional insert sections between the streamer sections is expensive and labor intensive.
A problem particular to determining the positions of towed seismic streamers with transmitters and receivers is keeping track of which transmitter's signal is being detected by which receiver at any given time. U.S. Pat. No. 4,187,492, “Device for Determining the Relative Position of Elongate Members Towed behind a Ship”, issued to Robert Delignieres and Mareil Marly on Feb. 5, 1980, discloses a method for employing different frequencies for different transmitters. Delignieres et al. '492 describes a system in which acoustic wave transmitters are positioned along a first streamer and acoustic pulse receivers are positioned along a second streamer. Each transmitter transmits at a different frequency and each receiver receives signals from only one transmitter. The system includes a telemetry system for determining travel time intervals of the acoustic pulses between transmitters and receivers and measuring the relative distances from these travel times. The transmitters and receivers primarily comprise a transducer six cylinders of piezoelectric ceramic of six different lengths vibrating in longitudinal mode at six different frequencies. Delignieres et al. '492 gives as an example six frequencies in the range of 20 to 100 kHz. However, employing different frequencies to distinguish the transmitters generally increases the complexity and expense of both the transmitters and the receivers.
U.S. Pat. No. 6,697,300 B1, “Method and Apparatus for Determining the Positioning of Volumetric Sensor Array Lines”, issued to Michael D. Holt on Feb. 2, 2004, discloses a method in which each transmitter transmits a distinguishable signal, even though the transmitters and sensors are of identical design, all operating at the same frequency. Holt '300 describes a system which comprises transmitters (including ceramic transducers), sensors (hydrophones) for receiving seismic reflections of the transmitters' signals from objects of interest, and detectors for receiving the transmitters' signals directly. The detectors determine travel times in terms of intervals of clock periods, and hence distances, between the streamers. The transmitters employ code division multiple access type pseudo-random numbers to uniquely identify the signals coming from each transmitter. The signals sent by the transmitters to the streamer-positioning detectors modulate a carrier wave outside the acoustic analysis band of the signals received by the object-positioning sensors, since Holt '300 teaches a system for sonar detection of enemy vessels which must be undetected by the other vessels. In the case of seismic acquisition surveys, this requirement would result in a transmitter carrier wave above the seismic acquisition band. These higher frequency signals suffer from higher attenuation, thus degrading the resolution required for positioning long streamer arrays.
U.S. Pat. No. 5,668,775, “Methods for Determining the Position of Seismic Equipment, and Applications of the Methods”, issued to Kjell Hatteland on Sep. 6, 1997, discloses a method employing acoustic transmitters between streamer section segments and conventional seismic receivers (hydrophones) located inside the streamers. Both power and communication to these transmitters go through the streamer harness. The transmitters operate at low frequencies, in the approximate range of 1 Hertz (Hz) to 1 kHz, encompassing the seismic frequency range. The transmitters generate a spread spectrum signal as an orthogonally encoded signal sequence with an unambiguous top in the form of a prominent peak in the signal's autocorrelation function. Cross-correlating the signal received by a receiver with the orthogonally encoded signal sequence of the transmitted spread spectrum signal allows the determination of a time difference between the detection of the signal by different receivers. This time difference, in turn, allows the determination of the distance between individual transmitters and receivers, based on a known in-line distance between receivers. A high number of transmitter-receiver combinations are used to determine a network which then gives the seismic equipment's geometrical configuration. However, in Hatteland 3 775, the seismic receivers are employed both for determining streamer positions as well as conventional seismic acquisition, instead of employing separate systems of dedicated receivers, leading to the same problems discussed above in Norton, Jr. et al. '682 and Langeland et al. '990.
The previously described methods for determining the positions of streamers contain a number of problems. In systems employing magnetic compasses, as in Neeley '111 and Gikas et al. '153, compass headings are referred to magnetic north, and knowledge about the local magnetic variation (declination) is necessary. The effect of magnetic storms and local anomalies can only partly be corrected for. Due to the sparse sampling along the cable and lack of information about how the streamer behaves between the compass locations, the positioning accuracy is not precise. This lack of precision is especially important when lateral steering devices used to control the cross-line position of the streamer cables are placed between the compass positions. Because compass readings are influenced by wave motion caused by weather conditions, the compass readings are filtered over time, with the result that the values used in the computations of streamer positions have a significant time lag.
Employing externally-mounted transceivers, either transmitters or receivers, as in Gikas et al. '153, Langeland et al. '990, and Norton, Jr. et al. '682, has several drawbacks. During streamer deployment and retrieval modes, attaching and removing the transceivers so that the streamers can be spooled directly on and off winches onboard the survey vessel requires considerable operational time, which is very expensive. Externally-attaching the transceivers (or any other equipment, such as magnetic compasses) to the streamer cables increases tow drag on the streamer and increases the noise in the detected seismic signals. The transceivers are also exposed to impact and thus transceivers are lost from time to time. In addition, the batteries have to be replaced at regular intervals. Because of the high frequencies used for the acoustic ranging, the performance may be degraded in hostile acoustic environments.
Locating transmitters or receivers in dedicated streamer inserts; between streamer sections, as in Austad et al. '302, is expensive and labor intensive. Additionally, both data redundancy and quality may be limited because of the limitation on spacing of the transceivers.
Employing transmitters designed to transmit at different frequencies, as in Delignieres et al. '492, may increase the cost of both the transmitters and receivers. Employing transmitters designed to transmit at higher frequencies, as in Holt '300, leads to signals which attenuate too rapidly over longer distances. This attenuation reduces the resolution at the longer distances employed in positioning long streamer arrays than in the shorter streamer arrays employed in detecting vessels, as in Holt '300.
Employing the seismic data acquisition receivers, instead of separate dedicated receivers, to determine steamer positions, as in Langeland et al. '990, Norton, Jr. et al. '682, and Hatteland '775, leads to problems. The acoustic position-determining network may be significantly degraded if the seismic acquisition receivers are not available on some streamers. Receiver unavailability may occur during streamer failure, maintenance or system testing.
Finally, the inability to make inline distance measurements, as in Norton et al. '682, leads to lack of precise knowledge of the distance between the transmitters and receivers when the streamer is under tow, which in turn, leads to lack of accurate knowledge of the amount of stretching of the streamer under tension. This lack of knowledge degrades the streamer position-determining accuracy.
Thus, a need exists for an improved method for determining positions of towed marine seismic streamers.